Lake and Reservoir Management 23:469-499, 2007 © Copyright by the North American Lake Management Society 2007 Fish and fish-eating birds at the Salton Sea: a century of boom and bust Allen H. Hurlbert1,a, Thomas W. Anderson2,3, Kenneth K. Sturm2,b and Stuart H. Hurlbert3 1Department of Biology, University of New Mexico, Albuquerque, NM 87131, USA 2Salton Sea Authority, Sonny Bono Salton Sea National Wildlife National Wildlife Refuge, Calipatria, CA 92233, USA 3Department of Biology and Center for Inland Waters, San Diego State University, San Diego, CA 92182, USA Abstract Hurlbert, A.H., T.W. Anderson, K.K. Sturm and S.H. Hurlbert. 2007. Fish and fish-eating birds at the Salton Sea: a century of boom and bust. Lake Reserv. Manage. 23:469-499. We reconstruct historical trends in both fish and fish-eating bird populations at the Salton Sea, California since the Sea’s formation in 1905. The fish community has undergone dramatic shifts in composition, from freshwater spe- cies present in the initial wash-in to stocked marine species to dominance by an unintentionally introduced cichlid. Historical catch records, creel censuses, and gill-netting studies suggest that total fish biomass in the lake increased dramatically throughout the 1970s, crashed in the late 1980s, recovered in the mid-1990s, and crashed again in the early 2000s. We speculate that crashes in fish populations are primarily due to three physiological stressors—ris- ing salinity, cold winter temperatures, and high sulfide levels and anoxia associated with mixing events. The trends in fish biomass are mirrored in population trends of both breeding and wintering fish-eating birds, as indexed by a number of independent bird surveys. In contrast, most non-piscivorous birds at the Salton Sea show no indication of these temporal dynamics. We discuss possible explanations for the observed trends in fish-eating bird populations, including decline in organochlorine insecticide use and fluctuations in prey abundance at the Salton Sea. Finally, we speculate on the future prospects of both fish and fish-eating birds at the Salton Sea given the environmental chal- lenges the Sea currently faces. Key words: Aechmophorus occidentalis, Ardea herodias, Aythya affinis, Casmerodius albus, Christmas Bird Counts, Circus cyaneus, DDT, Egretta alba, Egretta thula, Fulica americana, hydrogen sulfide, Hydroprogne caspia, long- term trends, Nycticorax nycticorax, Oreochromis mossambicus, organochlorine insecticides, Oxyura jamaicensis, Pandion haliaetus, Pelecanus erythrorhyncos, Pelecanus occidentalis, Phalacrocorax auritus, Podiceps nigricollis, Recurvirostra americana, saline lakes, salinity Introduction 1937; Dowd 1960; Hely et al. 1966; Cohen et al. 1999). For about 16 months the entire river flowed into a below sea level In the spring of 1905 the Colorado River broke through a depression known as the Salton Sink located on the northern temporary structure built for diverting water to agriculture in edge of the Colorado River delta. By February 1907, when the Imperial Valley in the southern California desert (Sykes the river had been diverted back into its regular channel by a massive rock-dumping effort by the Southern Pacific Rail- 2 a Current address: National Center for Ecological Analysis and road, the sink held a 1348 km lake, the Salton Sea. Synthesis, 735 State St., Suite 300, Santa Barbara, CA 93101, USA, [email protected] In April 1908, the later famous California naturalist, Joseph b Current address: Canaan Valley National Wildlife Refuge, Grinnell, with Chas. Richardson, Jr. and a boatman called U.S. Fish and Wildlife Service, HC 70 Box 200, Davis, WV “Donham,” set out to survey the bird life of this new water 26260, USA 469 Hurlbert, Anderson, Sturm and Hurlbert body (Grinnell 1908). They launched a small motorboat at the Methods and data sets northern end of the lake and traveled nine hours southwest- ward, covering about 65 km (40 miles) before attaining their The data and information sources that form the basis of this first objective, Echo Island. During the trip they observed paper, together with our methods of analyzing them, are as Eared Grebes, Western Grebes, Common Loons, Double- follows. crested Cormorants, Caspian Terns, and Ring-billed Gulls (scientific names of all bird and fish species mentioned are Fish and environmental data given in Appendix 1). Landing on Echo Island they found 980 White Pelican nests with eggs and others under construc- Interpretation of trends in fish-eating birds would be facili- tion thus documenting the southernmost recorded nesting tated by information on how the kinds, population densities, colony of the species. No other bird species were observed and sizes of fish in the Salton Sea have changed over time. nesting there. The following day they visited Pelican Island, Unfortunately, hard information of this sort is mostly lack- “a small rocky ledge three miles from Echo Island”, and ing. Aside from the detailed studies of the 1950s (Walker recorded egg-occupied nests of Double-crested Cormorant 1961) and recent gillnet surveys (Costa-Pierce and Riedel (their “Farallone cormorant”) (147 nests), Great Blue Herons 2000; Riedel et al. 2002, 2007; Caskey et al. 2007), most (7 nests) and White Pelicans (3 nests). Donham claimed that historical information on the fish populations is found in a “two and three years ago” (i.e., presumably in the springs of few unpublished reports of creel censuses (Hulquist 1981; 1905 and 1906), when the Salton Sea was still filling, this Black 1988), earlier gillnet surveys (Black 1979-1980, 1982, Pelican Island was “the nesting ground of the pelicans.” 1988), and monitoring of the planktonic eggs and larvae of fish (Matsui et al. 1991). The popularity of the Salton Sea Thus from its earliest days, the Salton Sea seems to have sport fishery, however, has resulted in many articles in the been an attractive feeding and nesting area for piscivorous popular press, many references to fishing conditions and waterbirds. Large numbers of fish undoubtedly were carried fish kills in unpublished records of the Salton Sea National into the lake as the Colorado River filled it. A few species, Wildlife Refuge (SSNWR) and Salton Sea State Recreation finding it a congenial environment—murky, warm year Area (SSSRA), and many memories in the minds of both round, nutrient-rich, and with few predators—were able to agency personnel and local residents. Individuals who have increase rapidly in abundance. Over the past century the been especially helpful in providing information include Salton Sea has undergone marked changes in its size, salin- Glenn Black, California Department of Fish and Game ity, fish assemblage, and other characteristics. This would seasonal aide (1971, 1972) and fisheries biologist (1978- lead one to predict that large changes in the bird assemblage 1983); Norman Niver, shoreline resident in Salton City associated with it have also occurred. With a present area of (1979-present); Steve Horvitz, successively Superintendent ca. 980 km2, the Salton Sea is the largest lake in California of the Salton Sea State Recreation Area and Director of State and has a salinity of about 47 g l-1. Parks Southeastern Region (1991-2000); James “Tex” Rit- ter, Chief Ranger, Salton Sea State Park (1975-1983); and The primary objectives of the present study are twofold. First, Mickey Jensen, staff member, SSSRA (1971-present). We we reconstruct the historical changes in the Salton Sea fish draw on all of these sources in our attempt to reconstruct fauna since the lake’s formation using all of the published historical change in fish abundance and assess its causes. data, unpublished creel censuses and gill net surveys, and Table 1 summarizes the sparse quantitative information anecdotal information available. Second, we examine and obtained since 1958 on composition of fish catches taken interpret trends in the abundance of fish-eating birds at the by hook and line and gillnetting. Figure 1 summarizes this Salton Sea since the 1950s using data from the Audubon and more anecdotal pre-1958 information on composition of Christmas Bird Counts, colonial nesting bird surveys con- the fish fauna. It also presents a frankly speculative estimate ducted by the Sonny Bono Salton Sea National Wildlife of how the total fish standing crop may have changed in the Refuge (1987-1999), and a monthly wildlife surveillance lake during 1905-2004. We have dared to indicate absolute program (2000-2004). Trends are related both to changes in values on the axis scaling standing crop, even in the absence Salton Sea fish populations and to regional environmental of a single empirical estimate of fish standing crop for the change and temporal patterns documented for the same bird lake. These values are not unrealistic given that for eutrophic species over western North America by the North American lakes in the temperate zone, standing crops of 130-430 kg ha-1 Breeding Bird Survey. We also speculate on the future of are typical (Carlander 1955), that the dominant fish, tilapia, the Salton Sea as habitat for piscivorous birds in light of is fairly omnivorous but feeds principally on phytoplankton, changes taking place there and elsewhere in the Colorado and that the lake is warm. The relative changes in abundance River delta region. over time, however, are those we will be attempting to relate to fluctuations in the fish-eating bird populations. 470 Fish and fish-eating birds at the Salton Sea: a century of boom and bust Figure 1.-Historical changes in fish biomass (A), fish species composition (B), and salinity (C) in the Salton Sea, 1907-2004. Salinity data for the years 1907-1916, 1923, 1929, and 1945-1996 are measured values, while other years reflect calculated values (Tostrud 1997, Watts et al. 2001). Fish biomass values are speculative, and information on species composition is from various sources (see text). 471 Hurlbert, Anderson, Sturm and Hurlbert Table 1.-Historical trends in composition of catch by fishermen (hook and line) and fish biologists (multi-panel gill nets), 1958-2004.